1. Field of the Invention
The present invention relates to a thermoplastic composition. More particularly, the invention relates to a slightly, or partially cured olefin thermoplastic elastomer composition and a method of producing the same. The elastomer composition has elasticity at room temperature, while it exhibits thermoplasticity at an elevated temperature. The composition, therefore, can be processed by methods conventionally used in the processing of thermoplastic resins.
The present invention also encompasses a thermoplastic composition obtained by blending the partially cured elastomer composition with an olefin plastic.
The invention further encompasses a method of producing a foamed product from either the partially cured elastomer composition or the thermoplastic composition obtained by blending the partially cured elastomer with an olefin plastic.
2. State of the Art
Injection molding of conventional rubber is carried out by blending additives with the rubber, kneading the blend and curing it after injection into a mold. This process entails such disadvantages as the necessity of using a special molding machine, the long cycle time involved and the need to carry out a number of complicated steps. Similar disadvantages are incurred in extrusion molding, and these disadvantages have made it impossible to carry out smooth mass production of rubber products. Thus, it has been suggested that rubber might be replaced with materials which can be processed without curing but which have properties similar to those of rubber.
This suggestion has actually been put to practice with materials having rubber-like properties. Among the materials which have been used are soft plastics such as soft vinyl chloride resin, ethylene-vinyl acetate compolymer, and low density polyethylene. Though these materials have good processability and high flexibility, they have such drawbacks as low heat-resistance and low rebound elasticity which severely limit their use.
In order to improve the heat-resistance and mechanical strength of soft plastics, it has been tried to blend them with a plastic of a high melting point such as high density polyethylene or polypropylene. This blending, however, does not result in a good product because it causes a loss of flexibility, and further, when a thick product is molded from the blended material sinkmarks are apt to occur. Recently, attention is being given to "thermoplastic elastomers", a group of materials having properties which fall between those of cured rubbers and soft plastics.
Olefinic thermoplastic elastomers are already known. For example, elastomers comprising mainly graft copolymer of polyethylene-butyl rubber or ethylene-propylene-non-conjugated diene rubber have been proposed. U.S. Pat. No. 3,806,558 discloses an olefinic thermoplastic elastomer comprising a polyolefin plastic and a partially cured rubber. Our tests show that this composition has good properties as a thermoplastic elastomer but that it has very poor fluidity compared to general purpose plastics, and therefore, when a thick or large product is injection molded, flowmarks appear on the product to such an extent that it is impossible to obtain a product of good appearance.
For the purpose of improving the fluidity of a composition comprising polyolefin plastic and partially cured rubber, one might consider:
(I) using a polyolefin plastic and/or a rubber of a low molecular weight, PA0 (II) employing a low degree of curing, and PA0 (III) using a higher percentage of polyolefin plastic in the blend. PA0 (a) 90 to 40 parts by weight of a peroxide-curable olefin copolymer rubber, PA0 (A) 100 parts by weight of a partially cured elastomer composition obtained by dynamic heat-treatment, in the presence of an organic peroxide, of a mixture of PA0 (B) 0 to 100 parts by weight of an olefin plastic.
The above (I) gives the composition decreased tensile properties, (II) results in low heat-resistance, tensile properties and rebound elasticity, and (III) results in decreased flexibility and gives the composition a tendency toward producing sinkmarks when it is used in molding thick products.
According to the examples described in the above mentioned United States Patent, the composition of a polyolefin plastic and a partially cured rubber disclosed therein is produced by a method comprising a first step wherein a mixture of a polyolefin plastic, a mono-olefin copolymer rubber and a curative is homogeneously kneaded at a temperature below the decomposition point of the curative, and a second step wherein the mixture is further kneaded at an elevated temperature above the decomposition point of the curative. This method involves many problems. The steps are conducted batchwise and operation temperature must be increased and decreased in the course of carrying out the steps so that a long time is required to complete the production process. The method is therefore not suitable for use in mass-production. If polypropylene is used as the polyolefin plastic, the operation temperature of the first step for homogeneous kneading should be above 165.degree. C., because the melting point of polypropylene is around 163.degree. C. On the other hand, the decomposition temperature of the curative, particularly if the curative used is an organic peroxide, will, at the highest, be 200.degree. C. This permits only a narrow temperature range in which kneading can be safely performed, and therefore, there will often occur scorching, that is, undesired curing of the mixture caused by premature decomposition of the peroxide before completion of the homogeneous kneading of the ingredients. One solution of this problem that might be considered is to first knead only the polyolefin plastic and mono-olefin copolymer rubber before adding the curative at a low temperature, and subsequently, to knead the mixture at a temperature above the decomposition point of the curative to cause a slight crosslinking reaction.
This procedure, however, raises another problem. Because the amount of the curative blended in is so small as to cause only slight crosslinking, and because the curatives usually used are solid at a normal temperature, it is difficult to disperse the curative homogeneously into the above mentioned mixture of the polyolefin plastic and the mono-olefin copolymer rubber. Thus, local crosslinkings occur to produce a composition having a heterogeneously crosslinked structure. Especially in the case where the polyolefin plastic is polypropylene, a similar problem arises in connection with the dispersion of a crosslinking promotor (preferably used with a peroxide curative so as to obtain a composition of good properties) further aggravating the problem of heterogeneous crosslinking. A heterogeneously crosslinked composition is inferior to one having a homogeneously crosslinked structure in such rubbery characteristics as tensile property, heat-resistance and permanent elongation.
It has been known to improve olefin plastics in such properties as wear-resistance, tear-resistance, impact-strength, anti-stress cracking property and flexibility by blending with a rubbery material as disclosed, for example, in Japanese Patent Publication No. 6538/1959. Also, it has been known to further improve the above mentioned properties by using a cured rubber, as disclosed in Japanese Patent Publication Nos. 11240/1961, 2126/1963 and 21785/1966. In general, however, rubber has lower fluidity than olefin plastic, and the compatibility of rubber with olefin plastic is not so high. Consequently, although products produced from olefin plastic with which rubber is blended are free of sinkmarks, they are susceptible to the occurrence of flowmarks and are therefore of inferior appearance. This problem becomes more significant if the olefin plastics are blended with cured rubber.
One method practiced in producing foamed products of elastomers involves kneading natural or synthetic rubber with a curative and a foaming agent, processing the kneaded composition into a desired shape and to heat the composition to cure and foam it.
In this method, the rubber is cured before foaming takes place. Because the procedure is complicated and a special heating apparatus is necessary, and further, because the curing and foaming step takes a long time, this method has not been effectively industrialized.
In recent years, efforts have been made to replace rubber with foamed soft olefin plastics such as ethylene-vinyl acetate copolymer and low density polyethylene. However, foamed soft olefin plastics have only limited use because of two major drawbacks: they have lower heat-resistance and, because of their lower tension at melt, are susceptible to the formation of coarse cells or traces of foam collapse on the surface. For the purpose of eliminating such drawbacks, it has been proposed to cause foaming after crosslinking by irradiation with high-energy rays or by chemical crosslinking agents. Such methods require additional special steps and special devices, and therefore, are as disadvantageous from the economic point of view as the above mentioned conventional method of producing cured-foamed rubber products.
On the other hand, from, for example, the above mentioned United States Patent, it is known that a partially cured composition comprised of olefin copolymer rubber and olefin plastic can be used as a thermoplastic elastomer exhibiting properties falling midway between those of soft olefin plastic and cured rubber.
According to our experiments, such a composition has poor tension at melt and, since foam produced in the composition tends to collapse, the highest ratio of expansion of a foamed product obtained therefrom is 1.2. Furthermore, the cells are unevenly distributed and lack uniformity of size, and the surface of the product suffers from coarseness due to foam-collapse. These problems cannot easily be overcome by changing such a processing conditions as the extruding temperature and the decomposition point of foaming agents.
Also, those skilled in the art are aware of the fact that the foam-processability of olefin plastic can be improved by blending in rubber to decrease the temperature dependency of the viscosity at melt of the olefin plastic, and to improve such properties of the foam products as wear-resistance, tear-resistance, impact-strength, anti-stress cracking property and flexibility. However, in blending non-cured rubber with olefin plastics, it is necessary to use a high blend ratio of the rubber because the viscosity at melt of the non-cured rubber is substantially temperature dependent. As a result, there is a degradation of heat-resistance. On the other hand, cured rubbers have lower fluidity than olefin plastic and their compatibility with plastic is not so good. Thus, the blend of an olefin plastic and a cured rubber gives foamed products having non-uniform cell size and poor appearance.